The invention relates to a high-frequency receiver comprising a first oscillator, a second oscillator, and a pair of signal paths connected in parallel to an input. The signal paths each include, arranged in succession, the series arrangement of a first pair of mixing stages, receiving from the first oscillator signals in a phase-quadrature relationship for a down-conversion of the received high-frequency signals; and filtering means for selecting a received high-frequency signal and a second pair of mixing stages, receiving from the second oscillator signals in a phase-quadrature relationship having a frequency value f.sub.2 for an up-conversion of the selected high-frequency signal. The receiver includes a super-position circuit connected to the second pair of mixing stages, and having a signal output for producing at the signal output an output signal containing two frequency components with the central frequency value f.sub.2 being situated substantially centrally between them; and a control signal arrangement comprising a third pair of mixing stages having first mixing inputs, second mixing inputs and outputs, the two second mixing inputs being connected to the signal output for generating at the outputs control-signals for correcting undesired amplitude and phase differences in the signal processing in the signal paths.
Such a receiver is known from the European patent application Ser. No. 122 657, to which U.S. Pat. No. 4,633,315 corresponds. This patent discloses how a received modulated high-frequency signal is processed in the two signal paths. As the down-conversion of the frequency is generally accompanied by a folding around 0 Hz of the frequency components in the modulation signal, a modulation diagram has been realized according to which an up-conversion of the frequency is effected of the signals in the two signal paths by means of the second pair of mixing stages. This frequency conversion is followed by a superposition (addition or subtraction) in the superposition circuit, for unfolding the above frequency components. Unfolding is accomplished when undesired frequency components in the output signal of the superposition circuit cancel each other substantially completely during the superposition process. However, mutual differences in the signal processing in the two signal paths an lead to undesired amplitude and phase differences, causing undesired frequency components to develop in the superposed output signal, thereby producing to distortion and whistling in the modulation signal obtained after detection of the output signal. If there is no modulation of the received high-frequency signal, after superposition there will remain mainly two frequency components in the output signal. The first component is the desired carrier component to be detected. The second component is one of the error components resulting from this superposition and is basically undesired, but forms a reliable measure for the possibly available undesired mutual difference in the signal processing in the two signal paths. In the known receiver having a narrowband carrier-selection circuit, only the desired carrier component of the output signal is allowed to pass together with the output signal. The carrier-selection circuit is connected in series with an additional phase-locked loop for regenerating from the carrier component an in-phase detection carrier used for deriving the control signals.
The disadvantage of utilizing such a narrow-band series-arrangement of the carrier-selection circuit and the additional phase-locked loop is the fact that fast phase-locking to a carrier having a different frequency, for example, when switching-over to a new signal to be received, is generally hard to accomplish.